1. Technical Field
The present invention relates to a power generation unit for taking out the charge, which is generated when a piezoelectric material such as a piezoelectric element is deformed due to an external force, as electrical energy, a method of controlling the power generation unit, an electronic apparatus and a transportation device including the power generation unit.
2. Related Art
When a piezoelectric material such as lead zirconium titanate (PZT), quartz crystal (SiO2), or zinc oxide (ZnO) is deformed, electrical polarization is induced inside the material, and positive and negative charges appear on the surface. Such a phenomenon is called a so-called piezoelectric effect. There has been proposed a power generation method of vibrating deforming member such as a cantilever to thereby make a weight repeatedly act on the piezoelectric material, and thus taking out the charge generated on the surface of the piezoelectric material as electricity using such a characteristic the piezoelectric material has.
For example, by vibrating a metal cantilever having a mass disposed at the tip and a thin plate made of the piezoelectric material bonded thereto, and taking out the positive and negative charges generated on the piezoelectric material due to the vibration, an alternating current is generated. Then, the alternating current is rectified by diodes, and then stored in a capacitor, and then taken out as electricity. Such a technology has been proposed in JP-A-7-107752 (Document 1). Further, there has been also proposed a technology of arranging that a junction is closed only in the period during which the positive charges are generated in a piezoelectric device to thereby make it possible to obtain a direct current without causing a voltage loss in the diodes (JP-A-2005-312269 (Document 2)). Since there is a possibility of downsizing the power generation unit by using these technologies, an application of, for example, incorporating the power generation unit in, for example, a small-sized electronic component instead of a battery.
In the power generation unit using such technologies, when a force stronger than the assumed force is applied to the cantilever to thereby excessively swing the cantilever, the cantilever might collide with a peripheral member to thereby be damaged. In order to prevent the above, there has been proposed a technology of providing an elastic body to an inside wall of a housing for housing the cantilever to thereby absorb the impact caused when the cantilever collides with the inside wall of the housing (JP-A-2003-218418 (Document 3)).
However, in the technology proposed in Document 3, since it is required to prepare a space for disposing the elastic body, there is a problem that it is difficult to sufficiently miniaturizing the power generation unit.